A light emitting device is a kind of semiconductor device that converts electrical energy into light by using a characteristic of a compound semiconductor. The light emitting device is widely used for home appliances, a remote controller, an electronic display board, a display device, a variety of automation apparatuses, and the like. A typical example of the light emitting device is a light emitting diode (LED).
The LED is operated in such a way that electrons and holes are recombined with each other while moving through a PN-junction by a forward voltage applied to a semiconductor of a specific chemical element, and the fall of an energy level due to the electron-hole recombination causes light emission to occur.
The LED is generally manufactured to have a very small size of 0.25 mm2 and is fixed using a mold. The LED has lead frames for applying a source voltage thereto and is mounted on a printed circuit board (PCB). A typical example of a LED package is a plastic LED package of 5 mm (T 1¾), and a variety of LED packages are being developed in specific application fields. Meanwhile, with the trend of miniaturization and slimness of an information communication device, various components of the device such as a resistor, a condenser and a noise filter are being further miniaturized. In accordance with this trend, the LED is also manufactured in the form of a surface mounted device (SMD) so as to be directly mounted on a PCB.
A color of light emitted from the LED is determined according to its wavelength obtained depending on a combination of elements constituting a semiconductor chip.
Recently, following red and green LEDs a blue LED has been developed, and the LED is being more widely used in various technical fields. For example, the LED is used as a white-light lamp or a light source for a display device. Moreover, the LED is further improved in its light luminance and thus is also used for an electronic display board and a camera of a mobile phone.
A general blue LED is constructed to include a sapphire substrate, a buffer layer formed on the sapphire substrate, an undoped GaN layer formed on the buffer layer, an N-type GaN layer formed on the undoped GaN layer, an active layer formed on the N-type GaN layer, and a P-type GaN layer formed on the active layer.
The active layer is a semiconductor layer made of luminescent material such as InGaN, and serves as a light-emitting region.
The P-type GaN layer is in contrast with the N-type GaN layer. When an external voltage is applied to the LED, electrons move from the N-type GaN layer into the active layer and holes move from the P-type GaN layer into the active layer. The electrons and the holes are then recombined with each other in the active layer to thereby cause light emission.
The abovementioned light emitting device has a drawback in that its reliability is degraded when its light luminance is improved, and vice versa. The light emitting device must have a high reliability so as to be used for a high-luminance electronic display board and a mobile phone. In order to improve the reliability of the light emitting device, each layer formed in the device must be improved in crystallinity. However, the light emitting device has limitations in reliability improvement because it has structural limitations for emitting light of a desired wavelength.
Also, when a high voltage is applied to the light emitting device so as to improve its light luminance, its active layer is frequently damaged by the applied high voltage, causing the further degradation of its reliability.
Furthermore, when an electrostatic discharge (ESD) generated from electronic components near the light emitting device or from a human body is applied to the light emitting device, the light emitting device (specifically, its active layer) is frequently damaged by the applied ESD. To solve this problem caused by the ESD, an additional diode such as a Zener diode may be provided at a power input terminal of the light emitting device. However, this additional diode undesirably causes an increase in the manufacturing cost of the light emitting device.